Diet and trophic niche overlap of two Hylid species in a subtropical wetland of NE Argentina

Hyla nana and Hyla sanborni, recently recognized as two distinct species, are similar in morphology, and frequently coexist in the same reproductive sites. To determine whether food partitioning occurred during the reproductive phase, both numerical and volumetric analyses of the stomach contents were conducted on syntopic populations inhabiting a wetland system (Ibera Lake) located in the Corrientes Province, NE Argentina. This area is part of a large wetland system (Ramsar site no. 1162). The study was carried out during the wet season (December to February). The climate is subtropical with a high relative humidity. Specimens came from 9 sampling sites, and food items were identified and photographed to measure prey volume approximating their shape to that of a cylinder or a prolate spheroid. The specific trophic strategies were assessed using the Costello-Amundsen graphic techniques; prey niche breadth and niche overlap were also calculated. Factorial Analysis of the Correspondence (FAC) was applied to volumetric data in order to evaluate how the two species exploited the food resources. The average stomach content and food item volume were correlated to the size (SVL) of both species.A total of 123 stomachs were analysed: 47 of H. nana and 76 of H. sanborni. In terms of prey volume, H. nana showed a preference towards larger food items than H. sanborni. There was no difference between the two species in the mean stomach content volume of individuals, however the stomach contents of H. nana were slightly larger. The diets of Hyla nana and H. sanborni were similar in composition. All the analyses showed a marked preference towards Diptera in both species, particularly in H. sanborni. H. nana exhibited higher prey diversity than H. sanborni but, fewer prey items per stomach. The food spectrum composition, evidenced by the FCA, indicated a preference towards preys of large average size (Aranea and Hexapoda assemblage) for H. nana, whereas the distribution of H. sanborni was clearly linked with the Diptera. As predicted by the the Schoener selection strategy model for a two syntopic predator species system, Hyla nana, the larger species, selected prey bigger in size, while H. sanborni ate a larger number of prey specimens. Despite a high degree of trophic niche overlap, Hyla nana and H. sanborni did not segregate spatially, hence the coexistence mechanism, at least during the reproductive period, should not be related to competition processes.